The present invention relates to testing and calibration of electronic electricity meters. More specifically, the present invention relates to high speed multifunction testing and calibration of electronic electricity meters.
Calibration of electricity meters is typically verified by comparison of a meter under test (MTU) to a standard, or reference, meter for a time interval. The initiation of the time interval being signaled by the meter under test. The standard meter and MUT measure the same, or proportionate, electrical loads during the test interval. This has been the preferred way to test electricity meters for the last fifty years. With electromechanical single function electricity meters, the test interval corresponds to an exact number of revolutions of the meter disk for the MUT. With electronic meters, the test interval corresponds to some internal procedural step, or threshold. With either kind of meter, the test interval corresponds to a known quantum of electricity (known by the measurement of the standard meter) measured by the MUT.
Typically, prior to the start of the test interval, several seconds are allowed for loads to be established and meters to reach practically steady-state conditions; known as on the fly testing. Testing of individual elements, or stators, of polyphase meters has been carried out by a sequence of tests: one for each element and one for all elements combined. Additionally, the pulsating nature of a.c. quantities require extended test intervals to reduce their relative effects on the accuracy of comparisons where the MUT and the standard have different time characteristics, effective level of filtering, or smoothing. It is important to note, that a.c. power as watts integrated over time is sinusoidal and not a straight line. Many electronic meters track a.c. energy exactly, i.e., sinusoidally. However, many electronic standards used for meter calibration are filtered to give the appearance of smoothing out the sinusoidal. Comparing two such different devices can result in differences between their readings. This phenomenon was not an issue with electromechanical meters, as they inherently exhibited heavily filtered characteristics.
Referring to FIG. 1, herein, labeled prior art, the cyclic nature of a.c. power is shown. By way of example, two meters having equal accuracy: the first, a heavily filtered meter, integrates average power with respect to time, and the second, an unfiltered meter, integrates instantaneous power with respect to time. The energy curves of these meters are compared, e.g., during a time interval xe2x80x98axe2x80x99 to xe2x80x98bxe2x80x99.
The unfiltered meter records energy proportional to the cross hatched area. The filtered meter will record in proportion to the area bounded by the vertical lines xe2x80x98a/Txe2x80x99 and xe2x80x98b/Txe2x80x99, the abscissa, and the average power line, the broken line designated xe2x80x98Pxe2x80x99. In this example, the cross hatched area is obviously larger; for other values of xe2x80x98axe2x80x99 and xe2x80x98bxe2x80x99, it could be smaller or even the same.
Deviations that can occur in comparing meters are dependent on when in the voltage cycle, at xe2x80x98a/Txe2x80x99, the test starts and the test duration, defined by: (bxe2x88x92a)/T. As discussed hereinabove, the start is typically triggered by the meter under test (MTU) and usually not controlled. However, the prior art has addressed this problem by selecting a minimum comparison time which allows the limits of uncertainty to be managed.
By way of example, referring to FIG. 2, herein, labeled prior art, a plot is shown which is used to aid in selecting minimum test times with a desired accuracy. In one example, with an acceptable level of uncertainty of xc2x10.05%, the intersection on the 1.00 PF line is at approximately 5.3 seconds. Accordingly, the test time must be greater than 5.3 seconds at 1.00 PF at 60 Hz to obtain a 0.01% level of uncertainty in the comparison. Twice and four times as long are needed for similar uncertainty levels of 0.50% and 0.25 PF respectively. In general, with all the other variations unchanged, the longer the test interval the less the relative uncertainty.
The above-discussed and other drawbacks and deficiencies of the prior art are overconie or alleviated by the high speed multifunction testing and calibration of electronic electricity meters of the present invention. In accordance with the present invention, start and end test commands defining a test interval synchronized to whole cycles of the pulsating a.c. potentials and currents are signaled by a meter under test (MUT) to a plurality of standard meters, during which multiple electrical quantities are accumulated by both the MUT and the standard meters. The MUT accumulated quantities and the accumulated quantities of the standard meters are communicated to the external test device for comparison. These comparisons may be used by the external test device to calculate coefficients for correcting or calibrating the MUT. The coefficients are transmitted to the MUT where they are incorporated into the algorithms for calculation of the desired electrical quantities.
Accordingly, it is an important feature of the present invention that testing time is significantly reduce over the prior art method. Further, it is an important feature of the present invention that multiple testing functions can be accomplished during this short time interval due to the recording of the accumulations of electrical quantities during the test interval, in effect, one test replaces a sequence of prior art tests.
The following steps are followed in the electronic electricity meter during testing:
(1) receiving, at the MUT, a test ready and length of test message from an external test assembly; the length of test being defined by an integer number of whole cycles of a source;
(2) switching the electronic electricity meter from a communication mode of operation to a test/calibration mode of operation;
(3) clearing test accumulators in the MUT and the external teat device;
(4) sending a synchronized test start signal from the MUT to the external test assembly;
(5) separately accumulating at least one measured electrical quantity of the source for each of the MUT and the external test assembly until the length of the test is exhausted;
(6) sending a synchronized test end signal from the MUT to the external test assembly;
(7) switching the MUT from the test/calibration mode of operation to the communication mode of operation;
(8) receiving a send test data message at the MUT from the external test assembly;
(9) sending the at least one accumulated measured electrical quantity from the MUTI to the external test assembly; and
(10) verifying the at least one accumulated measured electrical quantity of the MUI with a corresponding at least one accumulated measured electrical quantity of the external test assembly.
The separately accumulated electrical quantities involved in a single test interval of multifunction electricity meters may be, but are not limited to, by element, or phase, and their totals: fundamental potential-squared hours, current-squared hours, Watthours, and varhours; fundamental plus harmonic potential-squared hours, current-squared hours, Watthours, and varhours; imputed neutral current-squared hours; apparent Volt-Ampere hours, and distortion Volt-Ampere hours.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.